1. Field of the Invention
The present invention relates generally to a method of and a system for controlling the air-fuel ratio of an air-fuel mixture to be burned in an internal combustion engine to a desired air-fuel ratio and particularly to a method and a system of this type by which the time required from provision of the air-fuel mixture to detection of a parameter representative of a function of the air-fuel ratio of the air-fuel mixture is reduced.
2. Description of the Prior Art
As is well known in the art, a technique has been recently developed which precisely controls the air-fuel ratio of an air-fuel mixture, fed into a combustion chamber of an internal combustion engine, by sensing with an exhaust gas sensor a parameter representative of a function of the concentration of a specific component in exhaust gases of the engine which component has a character corresponding to the air-fuel ratio of an air-fuel mixture burned in the engine, and by controlling the supply of fuel to the engine in accordance with an output signal representative of the sensed concentration of the specific component. This technique is applied to a carburetor as well as an electronically controlled fuel injection device. However, a carburetor has a general character that the air-fuel ratio variations between the products are fairly large. The air-fuel ratio variations between the products result in the exhaust emission variations between engines and is an obstacle to a strict control of the exhaust emission.
Thus, the accuracy of control and the inspection of the component parts of carburetors have been recently strikingly intensified so that the production cost of the carburetors is steadily increased.
However, even if the accuracy of control and the inspection of the component parts of the carburetors are strikingly increased in this manner, the air-fuel ratio variations between the products is intolerably great. On the other hand, for electronically controlling a carburetor so that the air-fuel ratio characteristics are uniform between the products, it is necessary to specially devise the way of controlling, the controlling parts, the electronic circuit and so on. This complicates the construction. Accordingly, it is a great problem of the electronically controlled carburetor which is to be solved to completely absorb the air-fuel ratio variations between the products.
However, when the above-mentioned technique is applied to a carburetor in which the air-fuel ratio variations between the products are large, the air-fuel ratio variations between the products are almost absorbed, and the air-fuel ratio is accurately controlled to a desired value in a relatively easy manner when the engine is in an operating condition in which the load varies narrowly. However, when the engine is in, for example, a rapid acceleration, the air-fuel ratio of an air-fuel mixture provided by the carburetor has already varied when a sensor provided in the exhaust system has sensed the concentration of a specific component in engine exhaust gases. As a result, since a control circuit generates an incorrect control signal, the confusion of control has occurred or the correction of control has been very much delayed.
This phenomenon is due to a delay by flowing of the air-fuel mixture in the carburetor and the intake passageway, a delay by the engine operations of intake, compression, explosion and exhaust, a delay by flowing of the engine exhaust gases from the engine to the sensor in the exhaust gas passageway, a delay in sensing of the concentration of the specific component by the sensor, and so on. From provision of the air-fuel mixture to detection of the concentration of the specific component, there is a substantial delay of nearly 0.2 seconds or 200 milliseconds at the vehicle speed of about 50 Km/h, in the case of, for example, an internal combustion engine for use in an automobile.
Also in the case of an internal combustion engine of an electronically controlled fuel injection type, a substantial delay is present which is shorter than the case of the engine including the carburetor mentioned above by several tens of milliseconds which correspond to the distance between the position of provision of a carburetor and the position of fuel injection in the intake passageway in the above-mentioned condition since fuel is injected at a position adjacent to the intake valve in the case of the engine of the fuel injection type.
As a solution to this problem, a system for controlling an air-fuel ratio of an air-fuel mixture provided for an engine has been proposed in which a part of the air-fuel mixture is extracted from the intake passageway into a combustion gas generator, the extracted air-fuel mixture is burned in the combustion gas generator to form combustion gases therein, a sensor senses a parameter representative of a function of the concentration of a specific component in the combustion gases which concentration is closely related to the air-fuel ratio of the extracted air-fuel mixture, and the air-fuel ratio of an air-fuel mixture provided for the engine is controlled to a desired value in accordance with the sensed parameter.
When the system proposed is applied to a carburetor, even if the carburetor is such that the air-fuel ratio variations between the products are present, the air-fuel ratio variations are absorbed so that the air-fuel ratio is corrected to a uniform desired value and even when the engine is in an operating condition in which load varies violently, the air-fuel ratio is satisfactorily corrected with a minimized delay.
However, in the conventional system, the resultant gases of the combustion gases are conducted into the exhaust gas passageway of the engine. As a result, the conventional system requires measures for maintaining the pressure of the combustion gases at a tolerably high level. This is to make combustion of the extracted air-fuel mixture possible in spite of a high back pressure in the exhaust gas passageway and to at all times maintain stable combustion of the extracted air-fuel mixture without being influenced by variations in the pressure of engine exhaust gases due to variations in engine load. Furthermore, when the extracted air-fuel mixture is not burned in the combustion gas generator due to a malfunction, or the like, the unburned air-fuel mixture is discharged to the atmosphere through the exhaust gas passageway to contaminate the atmosphere and at times the unburned air-fuel mixture causes a extraordinary combustion in the exhaust gas passageway to make the engine dangerous.